A boosted engine may exhibit higher combustion and exhaust temperatures than a naturally aspirated engine of similar output power. Such higher temperatures may cause increased nitrogen-oxide (NOX) emissions from the engine and may accelerate materials ageing, including exhaust-aftertreatment catalyst ageing. Exhaust-gas recirculation (EGR) is one approach for combating these effects. EGR works by diluting the intake air charge with exhaust gas, thereby reducing its oxygen content. When the resulting air-exhaust mixture is used in place of ordinary air to support combustion in the engine, lower combustion and exhaust temperatures result. EGR may also improve fuel economy in gasoline engines by reducing throttling losses and heat rejection.
In boosted engine systems equipped with a turbocharger compressor mechanically coupled to a turbine, exhaust gas may be recirculated through a high pressure (HP) EGR loop and/or through a low-pressure (LP) EGR loop. In the HP EGR loop, the exhaust gas is taken from upstream of the turbine and is mixed with the intake air downstream of the compressor. In an LP EGR loop, the exhaust gas is taken from downstream of the turbine and is mixed with the intake air upstream of the compressor.
HP and LP EGR strategies achieve optimum efficacy in different regions of the engine load-speed map. Moreover, each strategy presents its own control-system challenges. For example, HP EGR is most effective at low loads, where intake vacuum provides ample flow potential. At higher loads, it may be difficult to maintain the desired EGR flow rate. On the other hand, LP EGR provides adequate flow from mid to high engine loads, but may respond sluggishly to changing engine load, engine speed, or intake air flow. In gasoline engines especially, such unsatisfactory transient response may include combustion instability during TIP-out conditions, when fresh air is needed to sustain combustion but EGR-diluted air is present upstream of the throttle valve. Moreover, a significant lag in EGR availability can occur during TIP-in conditions, as the amount of EGR accumulated in the intake manifold may not be sufficient to provide the desired combustion and/or emissions-control performance.
It has previously been recognized that incorporating a second, supercharger compressor in a turbocharged engine system can help address flow potential and transient control issues as noted above. For example, U.S. Patent Application Publication 2009/0007563 describes a boosted diesel-engine system in which a supercharger is coupled downstream of a turbocharger compressor. The supercharger is operated in part to provide boost when the engine speed is relatively low and the turbocharger is incapable of providing the desired compression. In the disclosed systems, the EGR flow rate is controlled by varying supercharger and turbocharger boost, which affects the EGR flow potential. The supercharger is further used to actively pump the EGR when the engine speed is high and the turbocharger alone is capable of providing the desired compression. During such conditions, intake air flow from the turbocharger compressor is by-passed around the supercharger.
Despite its potential usefulness, the approach cited above suffers from at least one drawback. Specifically, inducted air and EGR are admitted to the supercharger through a common inlet, where pressure equalization can occur prior to compression. As a result, the rate at which EGR is supplied through the supercharger depends on the air pressure at the common inlet, which in turn depends on the level of boost provided by the turbocharger, the states of various by-pass and control valves, and other factors. Taking these dependencies into account may result in a complex EGR-control strategy.
The inventors herein have recognized that a specially configured supercharger compressor can be used to provide boost for an engine system and also enable improved and/or simplified EGR flow control. Accordingly, one embodiment provides a supercharger compressor comprising a plurality of rotors rotatably mounted in a housing, a first inlet for air, a second inlet for recirculated exhaust gas, and a flow separator arranged interior the housing. The flow separator is configured to form a slideable seal with one or more of the rotors. The slideable seal fluidically isolates the first inlet from the second inlet, at least in part, and retards pressure equalization therebetween. In this manner, undesired pressure equalization between inducted air and EGR upstream of the supercharger can be reduced or avoided, resulting in simplified EGR flow control.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted herein.